Thermoplastics are becoming increasingly popular for use in automobiles, housing, appliances, household goods and packaging. Among the materials which have been widely used for such purposes are high- and low-density polyethylene, polyvinyl chloride, polypropylene and polystyrene. While plastics made from these materials have been satisfactory, the raw materials used in their preparation are largely of petroleum origin. Since the demand for petroleum for all uses is ever increasing and since the supply is subject to depletion inventories, attempts are being made to develop thermoplastics from raw materials which are not dependent upon petroleum feedstocks. Also, in view of the growing concern of environmentalists over the disposal of waste materials, attempts are being made to develop thermoplastics which can be made from renewable resources and which are biodegradable to harmless substances. Still further, attempts are being made to develop thermoplastics which, if incinerated, will burn with a clean, blue flame.
Among the raw materials of nonpetroleum origin which have been used to produce thermoplastics are cyclic esters such as glycolide, lactide and the lactones. Glycolide, for example, has been polymerized to produce homopolymers which are reported to be useful in making prosthetic devices. Lactide has been polymerized to produce homopolymers which are reported to be useful in making adhesives, safety glass and finishes. Lactones have been polymerized to produce homopolymers which are reported to be useful in making moldings, coatings, fibers and films. In addition to the formation of homopolymers, the cyclic esters have been interpolymerized to form copolymers including terpolymers. Glycolide, for example, has been copolymerized with lactide and with various lactones to form copolymers which are reported to be useful in making absorbable sutures. Absorbable sutures are also reported to be obtainable when an optically active lactide, e.g., L-(-)-lactide or D-(+)-lactide is copolymerized with optically inactive D,L-lactide and when either L-(-)-lactide, D-(+)-lactide or D,L-lactide is copolymerized with other polymerizable cyclic esters having a 6- to 8-membered ring.
Homopolymers and copolymers of various cyclic esters such as glycolide, lactide and the lactones have been disclosed in numerous patents and scientific publications. See for example, U.S. Pat. Nos. 2,362,511; 2,683,136; 2,758,987; 3,021,309; 3,297,033; 3,463,158; 3,531,561; 3,620,218; 3,636,956; and 3,736,646; Canadian Pat. No. 863,673; British Pat. No. 779,291; German Pat. No. 1,153,902; Collected Papers of Wallace H. Carothers, Vol. 1, Interscience Publishers, Inc., New York (1940); F. J. van Natta et al, Jour. Amer. Chem. Soc. 56, 455 (1934); Ber. Deut. Chem. 36, 1200 (1903); W. H. Carothers et al, Jour. Amer. Chem. Soc. 54, 761 (1932); and K. Chujo et al, Die Makromolekulare Chemie 100, 262-266 (1967). In addition to the above patents and publications, U.S. Pat. No. 2,703,316 to A. K. Schneider, Mar. 1, 1955, POLYMERS OF HIGH MELTING LACTIDE, discloses that lactide homopolymers capable of being formed into tough, orientable, self-supporting thin films can be obtained by heating lactide having a melting point above 120.degree. C. to a temperature above the melting point of the lactide and below 200.degree. C. in the presence of a polyvalent metal oxide such as litharge. The patent to Schneider also discloses that orientable copolymers of lactide can be obtained by heating a mixture of lactide having a melting point above 120.degree. C. with glycolide, with tetramethyl glycolide, and with other polymerizable cyclic esters having a 6- to 8-membered ring (column 5, lines 10-15). According to the patent to Schneider, it is essential that the lactide have a melting point above 120.degree. C. (column 3, lines 50-53). If, for example, a lactide having a melting point of 118.degree. C. is employed, the resulting polymer is reported to be brittle and cannot be formed into drawable films (column 4, lines 58-60). The catalyst is a polyvalent metal oxide or a salt of such a metal, e.g., calcium, titanium, zinc, cadmium, antimony, magnesium and lead.
Canadian Pat. No. 808,731 to R. A. Fouty, Mar. 18, 1969, PREPARATION OF HIGH MOLECULAR WEIGHT POLYLACTIDES, discloses that lactide homopolymers can be obtained by heating L-(-)-lactide, D-(+)-lactide or mixtures thereof at a temperature of 20.degree. to 200.degree. C., preferably 90.degree. to 160.degree. C. in the presence of a divalent Group II metal hydrocarbyl catalyst such as diethylzinc, di-n-butylzinc and diethylmagnesium. The patent to Fouty also discloses that the lactides can be copolymerized with other polymerizable cyclic esters having a 6- to 8-membered ring, e.g., glycolide or tetramethyl glycolide (Page 2, lines 20-23). The polylactides as shown by the general formula in the Fouty patent (Page 1, line 24 and Page 7, line 11) and as described in the patent (Page 8, lines 2-3) contain the Group II metallic component of the catalyst in the form of a lactate. That the metallic component of the catalyst appears in the polylactides in the form of a metallic lactate is considered to be of significance in the preparation of sutures which require a high degree of non-toxicity (Page 8, lines 1-5).